The present invention relates to electronic parts and assemblies that utilize surface mount technology (SMT), and more particularly, to the surface mounting of printed circuit boards.
Difficulties in surface mount soldering of devices to circuit boards are well known. Some of the key factors that determine the nature and extent of these difficulties are the flatness of the circuit board, the co-planarity of the leads on the device, and the amount of solder required.
When solder paste is applied to a circuit board there needs to be physical contact between the paste and the leads on the device to be soldered in order to permit a proper solder joint to be formed after solder re-flow caused by heating. However, this imposes tight tolerances on the flatness of the circuit board and the co-planarity of the leads on the device. Presently the leads must be within approximately four thousandths of an inch of co-planarity. The thickness of the solder paste needs to be controlled extremely accurately, usually in the range of between six and eight thousandths of an inch. Since the xe2x80x9cflatnessxe2x80x9d of conventional circuit boards can vary as much as ten thousandths of an inch per inch, surface mount connections are usually only made over short distances.
Solder balls have been used to allow SMT devices to be manufactured with wider tolerance ranges as to co-planarity of their leads and to permit the use of circuit boards with wider tolerances with regard to flatness. When pre-applied to either a device or a circuit board, solder balls provide more solder per joint than can typically be supplied with solder paste. So-called ball grid array (BGA) devices have been developed that utilize rows and columns of discrete solder balls to make the required electromechanical interconnections upon solder re-flow. The result is that SMT has been successfully employed with solder balls over areas as large as approximately one and one-half inches square. A conventional BGA device 2 (FIGS. 1A and 1B) has solder balls 4 arranged in a grid pattern of rows and columns. Another conventional device 6 (FIGS. 2A, 2B and 2C) has a grid of balled pins 8. Typically conventional devices that utilize solder balls for attachment only have solder balls or balled pins located on one side and they have no other attachments because it is difficult to add balls or balled pins to a device that already has other components. When balls are added to pins by solder re-flow there must be some method of limiting the flow of solder or else the solder ball will substantially change its shape and thereby lessen its ability to accommodate tolerance variations. Therefore, at present, the type of devices that can be manufactured with balled pins is greatly limited.
There is a substantial need in the electronics industry to surface mount large products that contain other components. In the case of power supplies, for example, it is desirable to surface mount two parallel boards that overlap over a substantial area, e.g., two by four inches. It would also be desirable to surface mount two parallel boards in which one of the boards would have components mounted on both sides of the board. It would also be desirable to mount such large products to circuit boards with pins as well as solder balls but heretofore this has not been practical.
U.S. Pat. No. 6,189,203 describes the use of solder balls to interconnect two boards using surface mount technology on both boards in a power supply application. A problem with this scheme is that the same melting point solder is used for both contacts, since they derive from the common low melting point solder coating on the ball. This means that both the solder connection to the upper board and that to the lower board reflow during the solder reflow temperature, which may cause problems. Moreover, the tolerance in board flatness remains low, which normally cannot exceed the thickness of the thin solder coating on the round ball.
U.S. Pat. No. 6,137,164 describes the use of solder balls to interconnect a lower board using surface mount technology to a rigid interposer board, to the latter of which may be mounted additional components. This scheme suffers from the same disadvantages as described in connection with the earlier mentioned patent, with the additional disadvantage that an additional board is needed for the interconnect system.
The parent application, Ser. No. 09/122,225 (now U.S. Pat. No. 6,272,741), describes a system with some similarities to that of the ""164 patent in that an additional carrier board is needed but also requires the presence of a pin connector on the lower board for receiving a pin array on the carrier board to connect to the lower board.
An object of the present invention is a relatively simple construction for enabling an upper board to be surface mounted to a lower board and that also provides the advantage of increasing the permissible spread in board warpage and/or improved current-carrying ability.
In accordance with the present invention a surface mount contact is provided for attachment to a circuit board. The contact includes an elongate electrically conductive pin defining a shaft having a longitudinal axis and having an upper end and a lower end. A pre-formed heat re-flowable bonding member is attached to the lower end of the pin. An insulator surrounds the shaft of the pin intermediate the upper and lower ends and adjacent the pre-formed heat re-flowable bonding member.
The present invention also provides a circuit board assembly including an upper circuit board and a lower circuit board which are mechanically and electrically interconnected in spaced apart, parallel relationship by a plurality of electrically conductive pins. Each pin has a shaft with upper and lower ends. The upper ends of the pins are attached to the upper circuit board and the pins are arranged in a predetermined pattern. A plurality of separate discrete insulators each surround the shaft of a corresponding pin. The lower circuit board has a plurality of conductive pads arranged in the same predetermined pattern as the pins. A plurality of conductive joints are each formed by re-flow of pre-formed heat re-flowable bonding members previously attached to the lower ends of corresponding pins. Each conductive joint bonds a lower end of a corresponding pin and a corresponding conductive pad and forms an electromechanical connection therebetween.
It will be understood that the terms xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d are not meant to be limiting, but are merely used as a convenient way of identifying opposite ends of the pin or one of two boards. Reversal of the pin would result in the re-flowable bonding member being mounted at the upper pin end, as an example, and reversal of the boards with the pins in their original orientation would result in the upper pin end being mounted to the lower board, as a further example.
A preferred embodiment of our circuit board assembly includes upper and lower generally planar circuit boards held in a predetermined spaced apart relationship by a plurality of discrete electrically conductive pins. Each pin has a shaft with upper and lower ends. The upper ends of the pins are attached to plated through holes in the upper circuit board by a plurality of first solder joints. The pins extend from the underside of the upper circuit board in a predetermined pattern. A plurality of discrete insulators each surround and are permanently attached to the shaft of a corresponding pin. The lower circuit board opposes and is generally parallel with the upper circuit board. The lower circuit board has a plurality of conductive pads arranged in the same predetermined pattern as the pins extending from the upper circuit board. A plurality of second surface-mount solder joints are formed by re-flowing a pre-formed heat re-flowable bonding member attached to and surrounding the lower end of each pin. Each of the second solder joints bonds a lower end of a corresponding pin and a corresponding conductive pad. A first portion of the pins have lower ends that directly contact their corresponding conductive pads and a second portion of the pins have their lower ends spaced slightly above their corresponding conductive pads in a typical situation where the boards do not remain perfectly flat and parallel during the processing.
An alternate embodiment of our surface mount contact includes an elongate electrically conductive pin defining a shaft having a longitudinal axis and having an upper end and a lower end. A pre-formed heat re-flowable bonding member is attached to the lower end of the pin. An insulator with a conductive pad formed on an upper surface thereof surrounds the shaft of the pin adjacent the pre-formed heat re-flowable bonding member. In the preferred embodiment, the re-flowable bonding member is a solder ball surrounding the pin end. This pin with attached solder ball for convenience may from time-to-time be referred to as a pinned solder ball.